Additive color digital image display devices are well known and are based upon a variety of technologies such as cathode ray tubes, liquid crystal modulators, and solid-state light emitters such as Organic Light Emitting Diodes (OLEDs). In a common OLED color display device, a pixel includes red, green, and blue colored OLEDs. These OLEDs are color primaries that define a color gamut. By additively combining the illumination from each of these three OLEDs, i.e. with the integrative capabilities of the human visual system, a wide variety of colors can be achieved. However, it is not enough to simply provide an image. Displays are intended to provide a realistic representation of the images to the viewer, and there can be a need to correct display tonal responses to enhance the display image quality. The tonal enhancement must be implemented in the display's imaging chain.
The relationship between the reproduced luminances of objects in an image scene reproduction compared to the original scene luminances of those objects is a critical aspect of achieving viewer satisfaction with the reproduction. The theoretically correct reproduction for this purpose is a one-to-one relationship between the luminances of the original scene and the luminances of the reproduction only if the conditions under which the reproduction is viewed are identical to the conditions under which the original scene was viewed. This is described in the book “The Reproduction of Colour” by Dr. R. W. G. Hunt, (Fountain Press, England—Fourth Ed.), specifically in Chapter 6 wherein the fundamentals of tone reproduction are discussed.
In reality, displays showing reproductions of original scenes are viewed under conditions deviating significantly from the conditions under which the actual scenes are viewed. Consequently, actual tone reproduction produced by practical image reproduction systems must deviate significantly from a one-to-one relationship. In many instances, the preferred visual reproduction does not correspond to that matching the original scene most closely. The challenge in image scene reproduction, therefore, is to reproduce the original scene in such a manner that upon viewing the reproduction, the viewer has the impression of looking at the original scene. That is, viewing the reproduction should inspire in the viewer the same response as if viewing the original scene. The reproduction will thus appear natural to the viewer even though it may not be exactly faithful to the original scene, i.e. may not be an exact one-to-one luminance mapping relationship between the original scene and the reproduction. The result would then be a pleasing reproduction that would be preferred over reproductions not conveying that impression.
As is known in the art, the conditions under which an original scene or a display showing a reproduction of a scene are viewed can be characterized by a number of parameters, including illuminance on the scene or the display and surrounding illumination. Viewing conditions are typically further characterized by parameters of the major sources illuminating the scene or display. These include position of the sources, their chromaticity and illuminance, and their emission patterns (for example, specular or diffuse). One useful way of summarizing some of these parameters is by characterizing viewing flare: the amount of illumination not generated by a display which the user sees while viewing the display. This can include light reflected off the display from sources in the viewing environment, and light internally reflected within the display (i.e. optical crosstalk).
One example of a tone-reproduction standard is sRGB, described in IEC 61966-2.1:1999+A1.2003 Multimedia systems and equipment—Colour measurement and management—Part 2-1: Colour management—Default RGB colour space—sRGB, Version 1.10, Nov. 5, 1996. This standard attempts to account for the viewing conditions of a typical office environment. This is shown as system tonescale curve 170a in FIG. 3. This curve covers a reproduced luminance dynamic range of 3.5 decades over a three-decade range of scene luminance, which is typical for a CRT display.
Relative to previous displays, OLED displays provide a greater dynamic range that has not been used in previous tone-mapping methods such as sRGB. It is desirable, therefore, to provide an image reproduction system and method that offers an overall tone mapping in the reproduced image, as seen by the viewer, that is perceived to be a natural reproduction of the original scene while utilizing the full display dynamic range.